RETALIATION DLC CRASH FIX
Open Mod Manager, go to Mod Management on the top left, click ASI Mod Manager, scrolls down to "Retaliation Glitch Crash Fix" by Mgamerz, then press "Install ASI"
This one is for those poor souls who have the infamous RETALIATION DLC GLITCH (aka the glitch when your game crashes upon checking your DLCs). A big thanks to Mgamerz for ACTUALLY FIXING IT, as well as explaining the nature of the glitch and the reason why the workaround actually works.
What it is and why it happens?
It has to do with the Challenge system added with Retaliation DLC, and happens both on your and on EA Servers' side. It occurs when the DLC initializes and is fetching data from server about achievements (the things on the ticker), and if it hits exception, it endlessly gets stuck checking DLC. If your connection is slow, it'll happen when you click on MP, and then it goes up and crashes right away. So repairing or reinstalling your game, Origin, Windows, or buying a new PC altogether won't help.
FIX1
Using ME3 Mod Manager (if you don't have it yet, you can download it from Mgamer's site ME3Tweaks) https://me3tweaks.com/modmanager Enable first the Binkw32 ASI bypass from the menu on the top:
go to Tools > Bink Bypasses > Mass Effect 3 and click on Binkw32 there to activate it (you should see a checkmark if the activation is successful). Then from the menu on the top go to Mod Management and from the drop down menu select ASI Mod Manager, Select Mass Effect 3 Tab, scroll down and find Retaliation Glitch Crash Fix. If it says Not Installed, click on the Install ASI button on the right. The button should change to "Installed, Up to Date". You can always uninstall the mod should you wish to do so.
FIX2
This is a workaround if you chose not to use Fix1. Before launching your game, go OFFLINE status on Origin. Then launch your game, wait till it loads all DLCs and then go ONLINE status on Origin again. This will make it skip fetching the achievements of others/you, so it effectively eliminates the chance of it hitting the exception and crashing. You should be able to play the game without any problems now. THIS ALSO MIGHT WORK: try to remember what Challenge you had completed before this glitch plagued you, and redo it. Theoretically, this will rewrite your corrupt data and therefore the glitch will go away.